Method of manufacturing heat sensitive cable

ABSTRACT

A heat sensitive cable operable over a temperature range of between approximately -20° F. and 1650° F. The cable includes a tubular metallic sheath which is substantially temperature resistant and moisture impervious. It also includes a mass of compacted insulation material filling the sheath and having an insulation resistance variable with temperature in the range of between approximately 100 and 50,000 ohms. The cable further includes at least one thermoelectric conductor positioned within the insulation material filling the sheath. A method of manufacturing the cable includes the step of preparing the insulation material and advancing the thermoelectric conductor through a given region. It also includes the step of advancing a strip of flat metal material past apparatus for forming and welding a tubular sheath surrounding the given region. The method further includes the step of concurrently depositing the insulation material within the tubular sheath for enclosure and advancing the sheath, insulation material and conductor through apparatus for reducing the diameter of the sheath and applying tension to the conductor. With these steps, the insulation material is compacted and the conductor is positioned within the sheath.

This is a divisional of co-pending application Ser. No. 317,631 filed onNov. 2, 1981 now U.S. Pat. No. 4,491,822.

BACKGROUND OF THE INVENTION

The present invention relates to heat sensitive devices and, moreparticularly, to a heat sensitive cable and method of making same.

Heat sensitive cables which are characterized by the use ofsemiconductive materials having inverse temperature-resistancecharacteristics in conjunction with dissimilar thermoelectric conductorsare now well known in the art. Such constructions are particularlysuitable where it is desired to monitor the greatest temperatureexisting along the length of the cable, and are exemplified inconnection with a system for measuring and locating temperatureconditions of interest in U.S. Pat. No. 3,408,607. Thermister cableswhich are characterized by a core of semi-conductive material surroundedby a mass of temperature-resistant electrically-insulating materialcovered with a protective metallic sheath are also well known in theart.

Despite the clear advantages and many applications for such cables, theyhave simply not evolved to the point of providing the desired degree ofversatility. It has remained to develop a heat sensitive cable capableof generating a measurable and predictable voltage when the entirelength of cable is at ambient whether ambient be at -20° F., 1650° F.,or some value therebetween. If this could be achieved with an inversetemperature-resistance material, the thermoelectric output of the cableor a section thereof would be altered in a predictable fashion whensubjected to a temperature greater than ambient.

Moreover, if this could be achieved, the cable location where theincrease in temperature takes place could be located electronically.This could be achieved, for instance, as fully disclosed and claimed incopending U.S. Ser. No. 887,089 filed Mar. 16, 1978, now U.S. Pat. No.4,324,138, issued Apr. 13, 1982, for a method of and apparatus andsystem for determining temperature conditions. As set forth therein, theapplications are virtually limitless.

While the value of heat sensitive cable has long been recognized, it hasremained to provide such a cable having the requisite versatility forthe many applications to be benefited by use thereof. In fact, despitemy many prior inventions in this field, as exemplified by U.S. Pat. Nos.3,408,607 and 3,513,432, the missing link to providing a highlyversatile cable has remained. Despite the advantages that will berecognized by those skilled in the art, heat sensitive cable which isoperable over a temperature range of between approximately -20° F. and1650° F. has simply not been available.

It is therefore an object of the present invention to provide a heatsensitive cable operable over a temperature range of betweenapproximately -20° F. and 1650° F.

It is also an object of the present invention to provide a cable of thetype described utilizing a material having an insulation resistancewithin the indicated temperature range variable with temperature in therange of between approximately 100 and 50,000 ohms.

It is a further object of the present invention to provide a cable ofthe type described which comprises a thermocouple temperature monitoringdevice having a metallic tubular sheath containing two dissimilar metalthermocouple wires packed in a semiconductive ceramic powder.

It is another object of the present invention to provide a cable of thetype described wherein the thermocouple wires surrounded bysemiconductive ceramic powder are spaced equidistant from each other andthe outer sheath.

It is still another object of the present invention to provide a cableof the type described which is passive and self-generating to generate avoltage potential between the thermocouple wires indicative of thetemperature existing along its entire length or at the hottest pointalong its length if the temperatures are unequal.

It is still another object of the present invention to provide a cableof the type described capable of precise, non-perishable, reproduciblemeasurement of the temperature and identification of the location of thehottest spot when monitoring with a high input impedance temperaturedevice.

It is another object of the present invention to provide a cable of thetype described wherein the tubular sheath and the thermocouple wires canbe formed of various materials and combinations of materials to yieldvarious mechanical properties and temperature-voltage response curves.

It is a further object of the present invention to provide a cable ofthe type described which can be produced in lengths of thousands of feetat a fraction of the cost of making other types of constructions of heatsensitive cable.

An additional object of the present invention is to provide a cable ofthe type described utilizing commercially available materials andprocesses to manufacture the cable.

These and other objects, advantages and feature of the present inventionwill be apparent from a consideration of the accompanying specification,claims and drawings.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a heat sensitive cableoperable over a temperature range of between approximately -20° F. and1650° F. The cable includes a tubular metallic sheath which issubstantially temperature resistant and moisture impervious. It alsoincludes a mass of compacted insulation material filling the sheathwhich has an insulation resistance variable with temperature in therange of between approximately 100 and 50,000 ohms. The cable furtherincludes at least one thermoelectric conductor positioned within theinsulation material filling the sheath. With this construction, thecable has the requisite versatility for use over an extremely widetemperature range to serve the many applications encountered today.

In a preferred embodiment, the insulation material comprises manganesedioxide heated in a vacuum furnace at a temperature of approximately1650° F. Preferably, the manganese dioxide is heated for a period oftime of between approximately 3 and 10 minutes with the furnace beingdrawn to a vacuum of approximately 500 microns of mercury or less.Moreover, the vacuum furnace is advantageously preheated for a period oftime of approximately 15 minutes after the manganese dioxide has beenplaced in the furnace at a temperature of approximately 1250° F. priorto raising the temperature to 1650° F.

With the manganese dioxide treated as described, the insulation materialhas an insulation resistance of between approximately 3,000 and 6,000ohms at approximately 72° F. when compacted to approximately 70% oftheoretical density within the tubular sheath.

While it is possible to construct a cable with only a singlethermoelectric conductor, it is usually advantageous to utilize a pairof thermoelectric conductors positioned within the insulation materialfilling the sheath. One of the conductors is preferably a wire ofnickel/chrome alloy and the other of the conductors is a wire ofcopper/nickel alloy, the nickel/chrome alloy comprising approximately90% nickel and 10% chrome and the copper/nickel alloy comprisingapproximately 55% copper and 45% nickel. Moreover, the sheath isadvantageously formed of either 304 or 304L stainless steel ornickel/chrome/iron alloy comprising approximately 75% nickel, 15% chromeand 10% iron.

With respect to the method of manufacturing the cable, the insulationmaterial is prepared having an insulation resistance variable withtemperature in the range of between approximately 100 and 50,000 ohms.Next, at least one thermoelectric conductor is advanced through a givenregion lying generally forwardly and axially of the starting position.Then, a strip of flat metal material is advanced past apparatus forminga tubular sheath surrounding the given region. Next, a sufficient amountof the insulation material is concurrently deposited within the sheathto fill the formed sheath. Finally, the sheath, insulation materialcontained therein and the conductors are advanced through apparatus forreducing the diameter of the sheath and applying tension to theconductor. With this method, the insulation material is compacted andthe conductor is permanently positioned within the sheath.

In order to prepare the insulation material, manganese dioxide isadvantageously placed in a tube having closure means in both endsthereof. The closure means, such as plugs, are threadingly engaged withthe tube before placement of the tube in the vacuum furnace. Preferably,the plugs are tightened to compact the manganese dioxide in the tube andthe plugs are loosened one turn before placement of the tube in thevacuum furnace.

With regard to reducing the diameter of the sheath, the sheath issuitably drawn to a diameter no smaller than approximately 87% of theoutside diameter of the sheath as formed. It is then preferable tovacuum anneal the sheath after drawing for a period of time of betweenapproximately 5 and 15 minutes at a temperature of approximately 1650°F. If the sheath is to be reduced any further in diameter, the sheath issubsequently drawn and annealed after every 30% reduction in diameteruntil the diameter of the sheath has been reduced to a desireddimension.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a longitudinal cross sectional view of a heat sensitive cablein accordance with the present invention.

FIG. 2 is a cross sectional view taken on the line 2--2 of FIG. 1;

FIG. 3 is a longitudinal cross sectional view of a modified heatsensitive cable in accordance with the present invention.

FIG. 4 is a cross sectional view taken on the line 4--4 of FIG. 3;

FIG. 5 is a longitudinal cross sectional view of an apparatus forpreparing a material for insulation in the cables illustrated in FIGS. 1and 3;

FIG. 6 is a view similar to FIG. 5 showing the plugs loosened prior toheating; and

FIG. 7 is a longitudinal view of the apparatus of FIGS. 5 and 6 in avacuum furnace for heating of the material to be used as insulation inthe cables of FIGS. 1 and 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, and first to FIG. 1, the reference numeral 10designates generally heat sensitive cable in accordance with the presentinvention. The cable includes an elongated tubular metallic sheath 12which is substantially temperature resistant and moisture impervious. Italso includes a mass of compacted insulation material 14 which fills thesheath 12 and which has an insulation resistance variable withtemperature in the range of between approximately 100 and 50,000 ohms.The insulation material 14 is such that a measurable and predictablevoltage is generated over a broad temperature range. It is also afeature of the insulation material 14 that the measurable andpredictable voltage is indicative of temperature and is generatedcontinuously in the temperature range in a passive, self-generatingmanner. The cable further includes at least one elongated thermoelectricconductor means, such as the wire 16a, positioned within the insulationmaterial 14 filling the sheath 12. With these features of construction,the cable 10 is operable over a temperature range of betweenapproximately -20° F. and 1650° F.

In accordance with the invention, the insulation material 14 comprisesmanganese dioxide which has been heated in a vacuum furnace at atemperature of approximately 1650° F. The heating within the furnacetakes place for a period of time within the range of betweenapproximately 3 and 10 minutes with the vacuum furnace being drawn to avacuum of approximately 500 microns of mercury or less. Preferably, thevacuum furnace will have been preheated for a period of time ofapproximately 15 minutes after the manganese dioxide has been placed inthe furnace at a temperature of approximately 1250° F. prior to raisingthe temperature to 1650° F.

By treating the manganese dioxide in this fashion, the insulationmaterial 12 will have a final insulation resistance of betweenapproximately 3,000 and 6,000 ohms at approximately 72° F. Thesecharacteristics are particularly advantageous for measuring an increasefrom room temperature (ambient). As a result, the cable will generate avoltage differential in the presence of a localized temperature increaseover the ambient temperature or a general temperature increase ordecrease in the ambient temperature itself.

In the latter case, a conventional single point thermocouple can beincorporated to achieve still additional advantages. Specifically, thisresults in the production of a similar voltage increase for a similartemperature increase thereby providing the unique ability, e.g., topermit instrumentation to provide an alarm signal whenever an increasein local temperature exceeds the ambient by a preselected amount,regardless of the ambient temperature. In other words, comparing thevoltage output of the heat sensitive cable of the present invention withthe voltage output of a conventional single point thermocouple, it ispossible to provide automatic linear ambient temperature adjustment.

In the embodiment illustrated in FIG. 1, the cable 10 includes a pair ofelongated thermoelectric conductor means, such as wires 16a and 16b,positioned within the insulation material 12 filling the sheath 10. Itis advantageous for one of the wires 16a to be a wire of nickel/chromealloy and the other of the wires 16b to be a wire of copper/nickelalloy, with the nickel/chrome alloy comprising approximately 90% nickeland 10% chrome and the copper/nickel alloy comprising approximately 55%copper and 45% nickel. Moreover, the sheath 10 is preferably formed ofeither 304 or 304L stainless steel or nickel/chrome/iron alloy withapproximately 75% nickel, 15% chrome and 10% iron.

Referring to FIG. 2, it will be appreciated how the wires 16a and 16bare disposed in spaced and substantially parallel relation to oneanother and the sheath 12, and the wires 16a and 16b extend through atleast one end of the sheath 12 for connection to a suitable instrument(not shown) for measuring the voltage generated therebetween. Theinsulation material 14 completely surrounds each of the wires 16a and16b, separating and electrically insulating them from one another andfrom the sheath 12 and maintaining them in spaced and substantiallyparallel relation to one another and the sheath 12. However, since theinsulation resistance of the insulation material 12 is only in the rangeof between approximately 100 and 50,000 ohms, the cable 10 is capable ofgenerating measureable voltages across the wires 16a and 16b inaccordance with the well known Seebeck effect anywhere within the rangeof temperatures of between approximately -20° F. and 1650° F.

Referring to FIGS. 3 and 4, it will be appreciated that the cable 10'similarly includes a sheath 12' containing an insulation material 14'.The principal difference between the embodiment illustrated in FIGS. 3and 4 and the embodiment previously described in FIGS. 1 and 2 is thatthe cable 10' utilizes only a single thermoelectric conductor means,such as wire 16a', rather than a pair of wires, such as 16a and 16b, inthe earlier described embodiment. As will be appreciated by thoseskilled in the art, the cable 10' generates the measurable voltagebetween the wire 16a' and the sheath 12'.

With respect to the method of manufacturing the cable, the insulationmaterial is prepared having a compacted insulation resistance in finalform variable with temperature in the range of between approximately 100and 50,000 ohms. Next, at least one thermoelectric conductor means isadvanced from a starting position through a given region lying generallyforwardly and axially of the starting position. Then, a strip of flatmetal material is advanced past tubular sheath forming and welding meansso as to form a tubular sheath therefrom in a position surrounding thegiven region. Next, a sufficient amount of the insulation material isconcurrently deposited within the sheath to fill the formed sheath.Finally, the sheath, insulation material contained therein and conductormeans are advanced through means for reducing the diameter of the sheathand applying tension to the conductor means. With this method, theinsulation material is compacted and the conductor means is permanentlypositioned within the cable.

In order to avoid unduly lengthening the description herein, myinvention disclosed and claimed in U.S. Pat. No. 3,737,997 isincorporated by reference as fully teaching the means by which the cableof the present invention may be continuously manufactured. It will beappreciated, however, that the means by which the insulation material isprepared is not taught in that patent and the preparation of theinsulation material represents an important aspect of the presentinvention. Since it has not previously been possible to prepare aninsulation material having a compacted insulation resistance in finalform variable with temperature in the range of between approximately 100and 50,000 ohms and operable within the extremely broad temperaturerange of the cable herein, the specifics of preparing the material willbe set forth in some detail.

As previously mentioned, the insulation material comprises manganesedioxide heated in a vacuum furnace for a period of time of betweenapproximately 3 and 10 minutes at a temperature of approximately 1650°F. The manganese dioxide 18 is preferably placed in a tube 20 (see FIG.5) having closure means, such as plugs 22 in both ends thereof with theplugs 22 being threadingly engaged with the tube 20 before placement ofthe tube in the vacuum furnace 24 (see FIG. 7). As shown in FIG. 5, theplugs 22 are tightened to compact the manganese dioxide 18 afterplacement in the tube 20 and the plugs are subsequently loosened oneturn (see FIG. 6) before placement of the tube in the vacuum furnace 24.The manganese dioxide 18 has a grain size larger than any gap betweenthe mating threads of the tube 20 and the plugs 22. Otherwise, themanganese dioxide 18 could be drawn from the tube 20 into the vacuumfurnace 24 causing damage to the vacuum pump.

As previously described, the vacuum furnace is preferably drawn to avacuum of approximately 500 microns of mercury after placement of themanganese dioxide into the furnace. The vacuum furnace is advantageouslypreheated after the manganese dioxide has been placed in the furnace fora time of approximately 15 minutes and at a temperature of approximately1250° F. prior to raising the temperature to 1650° F. for the 3 to 10minute time period. By utilizing the parameters set forth, theinsulation material which results has an insulation resistance whencompacted in final form of between approximately 3,000 and 6,000 ohms ata temperature of approximately 72° F.

It will be appreciated that the diameter of the cable can be reduced bysuitable means such as drawing. The process contemplates the sheathbeing drawn to a diameter no smaller than approximately 87% of theoutside diameter of the sheath as formed after which the sheath isvacuum annealed for a time of between approximately 5 and 15 minutes ata temperature of approximately 1650° F. Subsequently, the sheath can bedrawn and annealed after every 30% reduction in diameter until thediameter of the sheath has been reduced to a desired dimension.

Depending upon the environmental conditions during the manufacturingprocess, the insulation material may absorb moisture. For this reason,the insulation material is preferably stored in a supply bin heated to atemperature of between approximately 220°-250° F. for deposit in theformed sheath. By utilizing the heated supply bin, it is possible toprevent moisture accumulation in the insulation material.

With the present invention, it is possible to provide an essentiallycontinuous heat sensitive cable, i.e., the cable can be produced inlengths of thousands of feet. Moreover, the heat sensitive cable of thepresent invention can be manufactured at a fraction of the cost ofmaking conventional types or constructions of heat sensitive cable. Aswill be appreciated, heat sensitive cable proposed in the past couldonly be made in short lengths utilizing labor intensive manufacturingmethods.

At present, the final chemical composition of the insulation material isnot known. It is known that pure manganese dioxide is an absoluteelectrical conductor. By heating the manganese dioxide in a vacuumfurnace drawn to a vacuum of 500 microns of mercury or less and heatedat approximately 1650° F. for a period of time of between approximately3 and 10 minutes after a preheat at 1250° F. for approximately 15minutes, a material is formed having a measurable insulation resistance,contrary to what might be expected, which is believed to be something inthe nature of Mn₂ O₃ but with a higher oxygen content. It is known thatthis results in an insulation material which when compacted in finalform has the desired resistance characteristics. Specifically, whencompacted in final form, the insulation material has a resistance withina temperature range of between approximately -20° F. and 1650° F.variable with temperature within a range of between approximately 100and 50,000 ohms.

With the present invention, the heat sensitive cable provides athermocouple temperature monitoring device which consists of a metallictubular sheath containing two dissimilar metal thermocouple wires packedin a ceramic powder which is a semiconductor. The wires, surrounded bythe ceramic powder, are spaced equidistant from each other andequidistant from the outer sheath. The cable is passive andself-generating to generate a voltage potential between the thermocouplewires which is indicative of the temperature existing along its entirelength, or if the temperatures are unequal, at the hottest point alongthe cable length when subjected to external temperatures. When monitoredby a high input impedance device, the heat sensitive cable is capable of(1) precise, non-perishable, reproducible measurement of the temperatureand (2) identification of the location of the hottest spot, and iscapable of utilizing varying combinations of materials to yield variousmechanical properties and temperature-voltage responsive curves.

Various changes coming within the spirit of the present invention maysuggest themselves to those skilled in the art. Hence, it will beunderstood that the invention is not to be limited to the specificembodiments shown and described or the uses mentioned. On the contrary,the specific embodiments and uses are intended to be merely exemplarywith the present invention being limited only by the true spirit andscope of the appended claim.

I claim:
 1. A method of manufacturing a heat sensitive cable operableover a temperature range of between approximately -20° F. and 1650° F.,comprising the steps of:providing manganese dioxide to be converted froma conductor into an insulation material having an insulation resistancewithin said temperature range variable with temperature in the range ofbetween approximately 100 and 50,000 ohms; heating said manganesedioxide in a vacuum furnace at a temperature of approximately 1650° F.to convert said manganese dioxide to said insulation material havingsaid variable insulation resistance within said temperature range;advancing at least one thermoelectric conductor means from a startingposition through a given region lying generally forwardly and axially ofsaid starting position; advancing a strip of flat metal material pasttubular sheath forming means so as to form a tubular sheath therefrom ina position surrounding said given region; concurrently depositing asufficient amount of said insulation material within the formed sheathto fill said tubular formed sheath; and advancing said sheath, saidinsulation material contained therein and said conductor means throughmeans for reducing the diameter of said sheath and applying tension tosaid conductor means to compact said insulation material and permanentlyposition said conductor means therein.
 2. The method as defined by claim1 wherein said manganese dioxide is heated in said vacuum furnace atsaid temperature for a period of time of between approximately 3 and 10minutes.
 3. The method as defined by claim 2 wherein said manganesedioxide is placed in a tube having closure means in both ends thereof,said closure means being threadingly engaged with said tube beforeplacement of said tube in said vacuum furnace.
 4. The method as definedby claim 3 wherein said closure means are plugs tightened to compactsaid manganese dioxide in said tube after placement therein, said plugsbeing loosened approximately one turn before placement of said tube insaid vacuum furnace.
 5. The method as defined by claim 4 wherein saidplugs and tube have mating threads sized such that any gap therebetweenis less than the grain size of said manganese dioxide being placed insaid tube for heating in said vacuum furnace.
 6. The method as definedby claim 2 wherein said vacuum furnace is drawn to a vacuum ofapproximately 500 microns of mercury or less after placement of saidmanganese dioxide into said furnace.
 7. The method as defined by claim 2wherein said vacuum furnace is preheated for a period of time ofapproximately 15 minutes after placing said manganese dioxide in saidfurnace at a temperature of approximately 1250° F. prior to raising thetemperature to 1650° F.
 8. The method as defined by claim 1 wherein saidinsulation material has an insulation resistance of betweenapproximately 3,000 and 6,000 ohms at a temperature of approximately 72°F. when compacted to approximately 70% of theoretical density withinsaid tubular sheath.
 9. The method as defined by claim 1 wherein saiddiameter reducing means is drawing, said sheath being drawn to adiameter approximately 87% of the outside diameter of said sheath asformed.
 10. The method as defined by claim 9 wherein said sheath isvacuum annealed after drawing, said sheath being annealed for a time ofbetween approximately 5 and 15 minutes at a temperature of approximately1650° F.
 11. The method as defined by claim 10 wherein said sheath issubsequently drawn and annealed after every 30% reduction in diameteruntil the diameter of said sheath has been reduced to a desireddimension.
 12. The method as defined by claim 1 including the step ofadvancing a pair of thermoelectric conductor means from said startingposition through said given region lying generally forwardly and axiallyof said starting position, said insulation material maintaining saidconductor means in spaced and parallel relation to each other and saidsheath, said conductor means both extending through at least one end ofsaid sheath for measuring said voltage.
 13. The method as defined byclaim 12 wherein one of said conductor means is a wire of nickel/chromealloy and the other of said conductor means is a wire of copper/nickelalloy.
 14. The method as defined by claim 13 wherein said wire ofnickel/chrome alloy comprises approximately 90% nickel and 10% chromeand said wire of copper/nickel alloy comprises approximately 55% copperand 45% nickel.
 15. The method as defined by claim 1 wherein said flatmetal material is a strip of 304 stainless steel.
 16. The method asdefined by claim 1 wherein said flat metal material is a strip ofnickel/chrome/iron alloy.
 17. The method as defined by claim 16 whereinsaid strip of nickel/chrome/iron alloy comprises approximately 75%nickel, 15% chrome and 10% iron.
 18. The method as defined by claim 1wherein said insulation material is stored in a supply bin heated to atemperature of between approximately 220°-250° F. for deposit in saidformed sheath.